In an internal combustion engine using diesel as a fuel, a fuel injection pump compresses the fuel into high pressure and delivers the fuel to an injector installed at a combustion chamber. An injection device for substantially compressing and delivering the fuel includes a plunger and a barrel. The injection device compresses and delivers the fuel when the plunger serving as a piston reciprocates in the barrel serving as a cylinder.
A configuration of the injection device including a plunger and a barrel will be described with reference to FIGS. 1 and 2. A plunger 100 is inserted into a barrel 200 to slidably reciprocate in the axial direction (namely, in the vertical direction).
The plunger 100 is operated to reciprocate by a cam of a cam shaft (not shown) installed at the injection pump. A relief groove 102 communicating with a plunger chamber 202 and a control edge 104 communicating with the relief groove 102 are formed at the plunger 100.
The barrel 200 has a plunger chamber 202 and a fuel feeding/distributing chamber 204 formed at its inside and outside, respectively, and a spill port 206 for communicating the plunger chamber 202 with the fuel feeding/distributing chamber 204 is formed at the barrel 200.
In FIGS. 1 and 2, when the plunger 100 descends so that its upper surface is located below the spill port 206, a fuel flows through the spill port 206 into the plunger chamber 202, and the fuel starts being compressed from the point when the plunger 100 ascends so that its outer circumference closes the spill port 206. If the pressure reaches a predetermined level, a delivery valve at the upper portion of the plunger chamber 202 is opened so that the compressed fuel is transferred to the injector.
Subsequently, if the plunger 100 ascends further so that the control edge 104 encounters the spill port 206, the high-pressure fuel in the plunger chamber 202 leaks through the relief groove 102 and the control edge 104 to the spill port 206, thereby releasing pressure.
As described above, in the fuel compressing and releasing procedure, the process of compressing the fuel over about 800 bars and releasing the pressure to about 3 bars is periodically repeated.
Here, since the fuel pressure is relieved by the spill port 206, at the instant that the spill port 206 is opened, a high-speed fuel flow occurs due to a great pressure difference as described above, and accordingly the rapidly flowing fuel collides with the wall of the spill port 206, which causes erosion.
In addition, if the static pressure of the fuel is lowered due to the high-speed flow of the fuel to be equal to or lower than a vapor pressure, cavitation phenomenon which generates minute bubbles occurs. With the relief pressure, these bubbles burst at the outer circumference of the plunger 100, the inner surface of the barrel 200 and the surface of the spill port 206. Therefore, the cavitation erosion occurs at the surfaces of the plunger 100, the plunger chamber 202 and the spill port 206, which becomes a factor of pressure leakage and deteriorates the durability of the injection device.